孟加拉湾若开褶皱带晚新生代构造特征初步研究

A Preliminary Study on the Late Cenozoic Structural Characteristics of the Arakan Fold Belt , Bay of Bengal

孟加拉湾若开褶皱带位于印度-缅甸山脉西部山前,由NNW—SSE向带状分布的多排背斜构成,其构造特征研究仍然十分薄弱。本文通过钻井资料和二维地震反射剖面精细构造解析,尝试分析若开褶皱带晚新生代构造特征,重点关注若开褶皱带的滑脱层发育特征及背斜几何学及运动学特征,结果表明若开褶皱带发育多个滑脱层:①底部滑脱层,位于约6.5s(双程走时)处;②中部滑脱层,层位存在变化,可能位于第四系底部或上中新统下方约2.5s处。在区域挤压作用下,若开褶皱带发育与底部滑脱层和中部滑脱层相关的滑脱褶皱,构造变形主要受控于底部滑脱层,而中部滑脱层影响了局部构造变形。生长地层记录显示若开褶皱带构造变形自东往西迁移,变形前缘形成于第四纪。基于构造分析结果提出了若开褶皱带褶皱变形的两种运动学端元模型:模型1中不发育中部滑脱层,滑脱褶皱发育于底部滑脱层之上;模型2中发育中部滑脱层,滑脱褶皱发育于中部滑脱层和底部滑脱层之上,形成上、下两套构造层。若开褶皱带背斜几何学和运动学特征受下伏滑脱层控制,背斜在走向上叠置、分叉可能暗示着背斜下伏滑脱层在走向上发生了改变。流体超压可能是影响若开褶皱带构造变形的重要控制因素。

Arakan fold belt （ Bay of Bengal ） located at thewest piedmont of Indo-Burma Ranges consists of a series of NNW—SSE-trending anticlines , but its structural characteristics are very poorly studied. Integration of well data and 2D seismic reflection profiles was used to study the late Cenozoic structural characteristics of Arakan fold belt ,with a focus on the characteristics of detachment levels and geometry and kinematics of anticlines.Arakan fold belt is characterized by multiple detachment levels ：（ 1 ） basal detachment level ,which situates at about 6.5s （ Two-way travel time ）;（2 ） intermediate detachment levels ,which situates at the bottom of Quaternary or underlying upper Miocene at about2.5s.Those detachment levels generate two sets ofdetachment folds under regional compression. The basal detachments level controlled the development of Arakan fold belt ,while the intermediate detachment level affected the deformation locally.The deformation time recorded by growth strata shows that the deformation of Arakan fold belt propagates from east towest with the leading edge formed in Quaternary. Based on structural analysis resultswe propose two end-member kinematic models for folding in Arakan fold belt.Model 1 without intermediate detachment level ,detachment fold formed above basal detachment level ; Model 2 with intermediate detachment level , two sets of detachment folds developed above basal and intermediatedetachment levels , respectively.The wave length of anticline in Model 2 is much shorter than that of Model 1 , therefore , thewavelength of anticline can be used to judgewhether intermediate detachment level developed or not indeformed strata pile.The geometry and kinematics of anticline is primarily controlled by underlying detachment level （ s ）, the formation of overlapping anticlines or anticline bifurcation in Arakan fold beltwould imply the underlying detachment level （ s ） changed along strike.Fluid overpressure could be an important deformation controlling factor in formation of Arakan fold belt.